Algae as a Source for Biofuel Ruth B. McMichens

I pledge

Introduction As concern grows about the supply of fossil fuels, other sources of energy are

being sought. Biofuels have been one of the substances at the forefront of the discussion.

A number of sources for the production of biofuels have been considered. Biofuels are

fuels that are produced from living organisms or from metabolic byproducts. In order to

be classified as a biofuel, the fuel must contain over 80 percent renewable materials (1).

First generation biofuels are derived from edible biomass, primarily corn and soybeans in

the United States, and sugarcane in Brazil. These biofuels have a number of problems.

First, there is not enough available farmland to provide more than about 10 percent of the

developing countries’ liquid fuel needs. The use of first generation biofuels also raises

the price of animal feed and ultimately increases the cost of food. In addition, when the

total emissions of growing, harvesting, and processing corn are factored into the cost of

biofuel, it becomes clear that first generation biofuels are not very environmentally

friendly (2). Bioethanol is currently produced from corn and sugarcane while biodiesel is

being made from palm oil, soybean oil, and oilseed rape (3).

Second generation biofuels are made from cellulosic biomass. Sources include

wood residues like sawdust and other cellulosic sources like construction debris,

agricultural residues like corn stalks and wheat straw, fast growing grasses and woody

materials that are grown for the sole purpose of making biofuel. The advantage of second

generation biofuels is that they are abundant and do not interfere with the production of

food. Most of these energy crops can be grown on marginal lands that would not

otherwise be used as farmland (2).

Large amounts of cellulosic biomass can be sustainably harvested to produce fuel.

The U.S. Department of Agriculture and the Department of Energy have estimated that

the U.S. can produce at least 1.3 billion dry tons of cellulosic biomass every year without

decreasing the amount of biomass that is available for food, animal feed or exports. It is

estimated that more than 100 billion gallons of fuel could be produced from this amount

of biomass. It is also important to note that cellulosic biomass can be converted into any

type of fuel including ethanol, gasoline, diesel, and jet fuel (2).

Third generation biofuel includes fuel produced from algae and cyanobacteria.

Algae grown in ponds can be far more efficient than higher plants in capturing solar

energy especially when grown in bioreactors. If algal production could be scaled up to

industrial capacity, less than 6 million hectares would be needed worldwide to meet the

current fuel demand. This consists of less than 0.4% of arable land which would be an

achievable goal from global agriculture. For example, in Texas which has a land mass of

67,835,300 hectares, only 271,300 hectares would be required for the growth of algae (4).

In addition, many of the most efficient algal species are marine which means that no

freshwater would be necessary in the culture phase (3). Biofuel produced from algae is

an intriguing option. The potential of this option will be explored here.

Algae

Scenedesmus dimorphus (5)

Euglena gracilis (6)

Phaeodactylum tricornutum (7)

(8) Figure 1. Algae images. Algae are a diverse group of eukaryotic organisms that belong to the Phylum

Protista. These organisms use light energy to convert CO2 and H2O into carbohydrates

and other cellular products. During this process oxygen is released. Algae contain

chlorophyll a, which is required for photosynthesis. Many algae contain other pigments

that extend the range of light that can be used by these organisms for photosynthesis.

Organisms that are classified as algae are quite diverse. They include both microscopic

unicellular and macroscopic multicellular organisms. These organisms differ from other

eukaryotic photosynthetic organisms like land plants due to the fact that they lack an

organized vascular system and they have relatively simple reproductive procedures. As

one of the primary producers of carbohydrates and other cellular products, algae are

essential in the food chains of the entire world. A large portion of the oxygen in the

atmosphere is produced by algae (9).

Organisms that are considered algae are grouped together by a number of

properties. These include the main photosynthetic pigments of each group, the structure

of the cell wall, the type of storage products, the mechanisms of motility, and the mode of

reproduction. A number of algal groups derive their name from the major color

displayed by most of the algae in the group (9).

Algae are found anywhere there is water – fresh water, salt water, and in the soil.

Due to the fact that the oceans cover over 70% of the earth’s surface, aquatic algae are

major producers of oxygen and important users of carbon dioxide. Phytoplankton is

predominantly made up of unicellular algae. This phytoplankton is a major source of

food for many animals, large and small (9).

All algae are primarily made up of proteins, carbohydrates, fats, and nucleic acids

in varying proportions. While the percentages can vary with the type of algae, some

types of algae are made up of up to 40% fatty acids based on their overall mass. It is this

fatty acid that can be extracted and converted into biofuel (10).

Microscopic algae can be single celled organisms that are free floating or they can

be propelled by flagella. They can also grow in long chains or filaments. Other algae,

like Volvox, form colonies of 500 to 60,000 biflagellated cells which can be seen with

the naked eye (9).

Macroscopic algae are multicellular organisms that have a number of specialized

structures which serve specific functions. A number of varieties of algae have holdfasts

which are rootlike structures that mainly serve to anchor the organism to a firm substrate.

The stipe, or stalk of the algae, typically has leaf like structures attached to it. The blades

are the main photosynthetic part of the algae. Some large algae have air bladders that

allow them to maintain their blades in a position that allows for the maximum collection

of sunlight (9).

There are a number of species of algae that are being studied for their suitability

as crops for mass-oil production. Table 1 gives a list of these species. (11).

Table 1: Algae Strains Currently Being Studied

Neochloris oleoabundans Class Chlorophyceae Scenedesmus dimorphus Class Chlorophyceae. Preferred species for

oil production for biodiesel. Problem – produces thick sediment if not constantly agitated

Euglena gracilis Phaeodactylum tricornutum Diatom Pleurochrysis carterae Class Haptophyta Unicellular

coccolithophorid alga. Able to calcify subcellularly

Prymnesium parvum Toxic algae Tetraselmis chui Marine unicellular alga Tetraslmis suecica Isochrysis galbana Microalga Nannochloropsis salina (Nannochloris oculata)

Botryococcus braunii Can produce long chain hydrocarbons representing 86% of its dry weight

Dunaliella tertiolecta Oil yield of about 37%. Fast growing Nannochloris sp. Spirulina species

(11) Chlorophyceae, green algae, are the strain most favored by researchers. However,

green algae tend to produce starches instead of lipids and require nitrogen to grow. They

have the advantage that they have very high growth rates at 30oC and at high light levels

in aqueous solution. Bacilliarophya, diatom algae, are also favored by researchers. One

drawback is that the diatom algae require silicon to be present in the growth medium.

When algae are grown under nutrient deficient conditions, the algae produce more oils

per weight of algae, but the amount of algae produced was reduced. Most algae are

tolerant to temperature fluctuations, but diatoms have a narrow temperature range (11).

Lipids from Algae

Microalgae contain lipids and fatty acids as membrane components, metabolites,

storage products, and sources of energy. Microalgae which include algal strains, diatoms,

and cyanobacteria have been found to contain high levels of lipids - over 30%. Due to

the high lipid content, these microalgal strains are of great interest in the search for

sustainable sources for the production of biodiesel. Table 2 shows the chemical

composition of algae. It has been found that algae can contain between 2% and 40%

lipids by weight (11).

Table 2. Chemical composition of algae expressed on a dry matter basis (%) (10)

Strain Protein Carbohydrates Lipids Nucleic acid

Scenedesmus obliquus 50-56 10-17 12-14 3-6

Scenedesmus quadricauda 47 - 1.9 -

Scenedesmus dimorphus 8-18 21-52 16-40 -

Chlamydomonas reinhardtii 48 17 21 - Chlorella vulgaris 51-58 12-17 14-22 4-5

Chlorella pyrenoidosa 57 26 2 -

Spirogyra sp. 6-20 33-64 11-21 -

Dunaliella bioculata 49 4 8 -

Dunaliella salina 57 32 6 -

Euglena gracilis 39-61 14-18 14-20 -

Prymnesium parvum 28-45 25-33 22-38 1-2

Tetraselmis maculata 52 15 3 -

Porphyridium cruentum 28-39 40-57 9-14 -

Spirulina platensis 46-63 8-14 4--9 2-5

Spirulina maxima 60-71 13-16 6-7 3-4.5

Synechoccus sp. 63 15 11 5

Anabaena cylindrica 43-56 25-30 4-7 -

Algal oils have been found to be very high in unsaturated fatty acids. Some of

these unsaturated fatty acids that are found in different algal species include: arachidonic

acid, eicospentaenoic acid, docasahexaenoic acid, gamma-linolenic acid, and linoleic acid

(10). When comparing the lipid yield of algae to vegetable sources, algae can produce

between 20,000 and 100,000 liters per hectare. Table 3 shows the yields for selected

plants (12)

Table 3: Vegetable Oil Yields

Crop liters oil/ha US gal/acre Crop liters oil/ha US gal/acre

corn (maize) 172 18 camelina 583 62

cashew nut 176 19 sesame 696 74 oats 217 23 safflower 779 83 lupine 232 25 rice 828 88 kenaf 273 29 tung oil 940 100 calendula 305 33 sunflower 952 102 cotton 325 35 cocoa (cacao) 1026 110 hemp 363 39 peanut 1059 113 soybean 446 48 opium poppy 1163 124 coffee 459 49 rapeseed 1190 127

linseed (flax) 478 51 olive 1212 129

hazelnut 482 51 castor bean 1413 151 euphorbia 524 56 pecan nut 1791 191

pumpkin seed 534 57 jojoba 1818 194

coriander 536 57 jatropha 1892 202

mustard seed 572 61 macadamia nut 2246 240

(12)

Lipid accumulation in algae usually occurs during times of environmental stress,

including growth under nutrient deficient conditions. The lipid and fatty acid contents of

microalgae differ according to the culture conditions. It has been found that in some

cases the lipid content can be enhanced by imposing nitrogen starvation. Biochemical

studies have indicated that acetyl-CoA carboxylase (ACCase), a biotin containing

enzyme that catalyzes an early step in fatty acid biosynthesis, might be involved in the

control of the lipid accumulation process. In light of this fact, it might be possible to

increase lipid production rates by increasing the activity of this enzyme by genetically

engineering the microalgae (11).

Microalgae are classified as the most primitive form of plants. The mechanism of

photosynthesis in microalgae is similar to that in higher plants, but they are usually more

efficient converters of solar energy because of their simple cellular structure. Due to the

fact that the cells grow in aqueous suspension, microalgae have more efficient access to

water, CO2, and other nutrients. These factors account for the ability of microalgae to

produce larger quantities of oil per unit area of land as compared to terrestrial oilseed

crops (13).

Figure 2 illustrates the biosynthetic pathway by which lipids are created. Long

chain fatty acids are synthesized from acetyl CoA, malonyl CoA, and NADPH. As fatty

acid biosynthesis proceeds, a repeated series of reactions incorporates acetyl groups of

acetyl-CoA into an acyl group that is 16 to 18 carbons long. The enzymes that are

involved in this reaction are acetyl-CoA carboxylase (ACCase) and fatty acid synthase

(FAS). Acetyl-CoA carboxylase (ACCase) then catalyzes the conversion of acetyl-CoA

to malonyl-CoA. This is the committed step in the production of lipids. The name fatty

acid synthase is used to refer to a complex of a number of different enzymes that catalyze

the conversion of acetyl-CoA and malonyl- CoA into 16:0 and 18:0 fatty acids (14).

Figure 2. Fatty Acid Synthesis (14)

Botrycococcus braunii which is a green, colonial microalgae has been found to

produce unusual hydrocarbons and ether lipids. These hydrocarbons are classified as

n-alkadienes and trienes, triterpenoid botrycococcenes and methylated squalenes, as well

as a tetraterpenoid, lycopadiene. In addition to these compounds and classic lipids like

fatty acids, glycerolipids, and sterols, these algae synthesize several ether lipids closely

related to hydrocarbons (14).

Algae are of great interest in the production of biofuels due to the fact that a

number of species of freshwater and marine algae contain large amounts of high quality

polyunsaturated fatty acids which can be produced for aquaculture operations. Algae can

grow heterotrophically on cheap organic substrates, without light, and under well-

controlled cultivation conditions. Several strategies are important when determining

ways to increase the use of algae for commercial production of polyunsaturated fatty

acids in the near future. These include continued selection and screening of oleaginous

species, improvement of strains using genetic engineering, optimization of the culture

conditions, and the development of efficient cultivation systems. It is also important to

determine whether the polyunsaturated fatty acids are located within the membrane

lipids, or in the cytosol (15).

Algae can produce a large number of different types of lipids which include but

are not limited to, neutral lipids, polar lipids, were esters, sterols, and hydrocarbons, as

well as prenyl derivatives such as tocopherols, carotenoids, terpenes, quinones, and

phytylated pyrrole derivatives like chlorophylls (16).

When algae are grown under optimal conditions, they synthesize fatty acids

principally for esterification into glycerol based membrane lipids which make up about 5-

20% of their dry cell weight (DCW). Fatty acids include medium (C10-14), long chain

(C16-18) and very long chain (>C20) fatty acid derivatives. The major components of

the membrane glycerolipids are different kinds of fatty acids that are polyunsaturated and

are derived through aerobic desaturation and chain elongation from the precursor fatty

acids palmitic and oleic acids (16).

When there are unfavorable environmental or stress conditions for growth, many

algae change their lipid biosynthetic pathways toward the formation and accumulation of

neutral lipids (20-50% DCW), mainly in the form of triacylglycerol (TAG). TAGs,

unlike the glycerolipids found in membranes, do not perform a structural role but instead

serve mainly as a storage form of carbon and energy. There is evidence that suggests that

in algae, the TAG biosynthesis pathway may play a more active role in the stress

response, in addition to functioning as carbon and energy storage under environmental

stress conditions. After being synthesized, TAGS are deposited in densely packed lipid

bodies that are located in the cytoplasm of algal cells (16).

Potential Advantages and Challenges of Algae as Feedstocks for Biofuels

Many algal species have been found to grow rapidly and produce large amounts of

TAG. In light of this fact, it has been postulated that algae could be employed as cell

factories to produce oils and other lipids for biofuels and other biomaterials. There are a

number of potential advantages of algae as feedstocks. Algae can synthesize and

accumulate large quantities of neutral lipids. The growth rate of algal species is very

high. Some species have 1-3 doublings per day. Algae thrive in saline/brackish

water/coastal seawater for which there are few competing demands. Land that is not

suitable for conventional agriculture can be used to grow algal species. Algae utilize

growth nutrients like nitrogen and phosphorus from a number of wastewater sources

which also provides the benefit of wastewater bio-remediation. Algae can sequester CO2

from flue gases that are emitted from fossil fuel fired power plants and other sources

which reduce the emission of a major greenhouse gas. Other by-products including

biopolymers, proteins, polysaccharides, pigments, animal feed, fertilizer and H2 are value

added co-products or by-products that are produced from the growth of algae. Algae can

also grow throughout the year with an annual biomass productivity that surpasses that of

terrestrial plants by about tenfold (16).

Based on the photosynthetic efficiency and the growth potential for algae,

theoretical calculations indicate that an annual oil production of about 200 barrels of algal

oil per hectare of land may be achievable from mass algal cultures. Unfortunately,

production of biofuel from algae has not proceeded beyond the small laboratory or field

testing stage due to the fact that the lipid yields obtained from algal mass culture efforts

have, to date, performed at around 10-20 times lower than the theoretical maximum..

This fact has historically made algal oil production technology prohibitively expensive

(16).

In order for algae to be a viable source for biofuel on a commercial scale, many

fundamental biological questions relating to the biosynthesis and regulation of fatty acids

and TAG in algae need to be answered. It is important that physiological and genetic

manipulations of growth and lipid metabolism must be implementable, and critical

engineering breakthroughs related to algal mass culture and downstream processing are

needed (16).

Conversion of Lipids to Biofuel

Biodiesel which is produced by the trans-esterification, shown in figure 3, of

triglycerides with methanol produces the corresponding mono-alkyl fatty acid esters.

This biodiesel is an alternative to petroleum based diesel fuel. The properties of biodiesel

are mainly determined by the structure of the fatty acid esters that are present. The most

important characteristics include the ignition quality, how it flows in the cold, and its

oxidative stability. Saturation and the fatty acid makeup do not appear to have much of

an impact on the production of biodiesel by the trans-esterification process; they do have

an effect on the properties of the fuel product. It is interesting to note that saturated fats

produce a biodiesel with superior oxidative stability and a high ignition quality, but with

poor low temperature properties. Biodiesels that are produced using these saturated fats

are more likely to gel at ambient temperatures. If the biodiesel is produced from

feedstocks that are high in polyunsaturated fatty acids (PUFAs), the fuel has good cold

flow properties, but they are susceptible to oxidation which leads to long term storage

problems (16).

Figure 3. Trans-esterification reaction (17)

Factors Affecting Triacylglycerol Accumulation and Fatty Acid Composition

Since the occurrence and the extent to which TAG is produced appears to be

species/strain-specific, and is ultimately controlled by the genetic make-up of the

organisms, algae produce only small amounts of TAG under optimal condition.

Synthesis and accumulation of large amounts of TAG occur when the cell is placed under

stress conditions that are imposed by chemical or physical environmental stimuli, either

acting individually or in combination. The major chemical stimuli include nutrient

starvation, salinity, and growth medium pH. The major physical stimuli include

temperature and light intensity. In addition, the age of the culture affects the TAG

content and the fatty acid composition (16).

Nitrogen limitation is the single most critical nutrient affecting lipid metabolism

in algae. A general trend towards the accumulation of lipids in response to nitrogen

deficiency has been observed in many species of various algal taxa. Silicon is equally

important in the lipid production of diatoms. It has been found that higher levels of TAG

are produced in silicon deficient diatoms. Other types of nutrient deficiency that promote

lipid accumulation include phosphate limitation and sulfate limitation (16).

Temperature has also been found to have a major effect on the fatty acid make-up

of algae. It has been found that as temperature decreases there is an increase in

unsaturated fatty acids. Likewise, when the temperature is increased there is an increase

in saturated fatty acids. Temperature has also been shown to affect the total lipid content

in algae. However, no general trend has been established yet (16).

Algae that are grown at different light intensities show remarkable changes in

their gross chemical composition, pigment content and photosynthetic activity. Usually,

low light intensity causes the formation of polar lipids, particularly the membrane polar

lipids that are associated with the chloroplast. However, high light intensity causes a

decrease in the total polar lipid content with a concomitant increase in the amount of

neutral storage lipids, mainly TAGs. The degree of fatty acid saturation can also be

altered by the intensity of the light. High light levels alter the fatty acid synthesis to

produce more of the saturated and mono-unsaturated fatty acids that mainly make up

neutral lipids (16).

Role of Algal Genomics and Model Systems in Biofuel Production

Since photosynthetic micro-organisms have the potential to produce 8-24 times

more lipids per unit area for biofuel production than the best land plants, these microbes

are in the forefront as future biodiesel producers. The nuclear genomes of only eight

microalgae have been sequenced to date. These include C. reinhardtii, Volvox carteri,

Cyanidioschizon merolae, Osteococcus lucimarinus, and Osteococcus tauris,

Aureococcus annophageferrens, P. tricornutum, and T. pseudonana. C. reinhardtii is the

only organism with extensive genomic, biological, and physiological data. For these

reasons, Chlamydomonas has become a model eukaryote microbe for the study of many

processes, including photosynthesis, phototaxis, flagellar function, nutrient acquisition,

and the biosynthesis and functions of lipids (16).

Genome sequence and biochemical studies have indicated that Chlamydomonas

is an ideal algal species for use as a source to produce biofuels. Chlamydomonas has an

extensive network of diverse metabolic pathways that can be manipulated using genetic

engineering as well as nutrient stress. The advantage of C. reinhardtii comes from the

fact that it can grow either photo-, mixo-, or heterotrophically while maintaining the

ability to perform photosynthesis. This allows researchers to study photosynthetic

mutations that are lethal in other organisms. It is interesting to note that C. reinhardtii

spends most of its life cycle as a haploid organism of either mating type + or -. This is an

advantage because it can be genetically engineered and single genotypes can easily be

generated. It is also feasible to genetically engineer C. reinhardtii to artificially over-

express fatty acids in order to increase the biofuel production (16).

Harvesting & Extraction of Algal Oil from Microalgae

Concentration of high density algal cultures is typically carried out by

concentrating the culture using either chemical flocculation or centrifugation. Chemicals

such as aluminum sulfate or iron (III) chloride are added to cause the cells to coagulate

and precipitate to the bottom or float to the surface. Then the algal biomass is recovered

by removing the supernatant or skimming the cells off the surface. Once this process has

taken place, the coagulated algae are no longer suitable as food for filter feeders due to

the increase in the particle size (18).

A cream separator is then used to centrifuge large volumes of the algal culture.

The type of algae being cultured determines the flow rate and the rate of centrifugation.

The cells are removed from the centrifuge and reconstituted in a small amount of water.

This suspension can be stored for 1 – 2 weeks in the refrigerator. The sample can also be

frozen, but it must be treated with glucose or dimethylsulfoxide. These chemicals act as

cryoprotective agents to maintain the integrity of the cells during the freezing process

(18).

Mechanical methods for the extraction of oil from algae include the

expression/expeller press and ultrasonic assisted extraction. When the

expression/expeller press is used, the algae are dried and the oil is pressed out with an oil

press. With ultrasonic assisted extraction, ultrasonic waves are used to create cavitation

bubbles in the solvent material. As these bubbles collapse near the cell walls shock

waves and liquid jets are created that cause the cell walls to break and release the

contents of the cell into the solvent. The simplest method for extraction is mechanical

crushing. Due to the different characteristics of algal strains, a number of different types

of press configurations including screw, expeller, and piston are used. In some cases,

chemical extraction methods are combined with mechanical crushing (19).

Organic solvents can be used to chemically extract the oil. Solvents that are

commonly used include benzene, ether and hexane. These chemicals are hazardous and

must be treated with care. The hexane solvent extraction method can be used alone or in

conjunction with the oil press/expeller method. After the oil has been extracted from the

algae using the expeller, the remaining pulp is then mixed with hexane in order to remove

any remaining oil. The hexane and oil are then separated by the use of a distillation

apparatus. When these two methods are combined, more than 95% of the total oil present

in the algae is extracted. Another method called soxhlet extraction uses hexane or

petroleum ether to remove the oils through a process of repeated washing in special

glassware. Another chemical method called supercritical fluid extraction uses liquefied

CO2 under pressure. The CO2 is heated to the point at which it has the properties of both

a liquid and a gas; this liquefied fluid then acts as a solvent in extraction in the oil (20).

Future of Algal Feedstock Based Biofuels

In order to better understand and manipulate algae for the production of biofuels,

it is important to undertake research to further elucidate the biosynthesis of algal lipids in

particular TAGs. It is clear that algae can provide the natural raw materials in the form of

lipid rich feedstock, but it is important to better understand the details of lipid metabolism

in order to effectively manipulate the process physiologically and genetically. To date

funding has been an issue for full scale development of algae based biofuel technology.

In order to fully exploit the potential of algae as a source for biofuels, it is necessary to

pioneer new engineering innovations and breakthroughs in algal mass culture as well as

downstream processing. However the most important issue is the need for research on

the fundamental biological questions that are related to the regulation of lipid metabolism

(16).

There are several biological challenges and opportunities. At the biochemical

level, it is important to understand the synthetic pathways in algae that are responsible for

the production of fatty acid and TAG. In addition, it is important to understand how algal

cells control the flux of photosynthetically fixed carbon and its partitioning into different

groups of major macromolecules. It is also necessary to understand the relationship

between the cell cycle and TAG accumulation. The isolation and characterization of

algae from unique aquatic environments is also necessary in order to provide insights into

the unique mechanisms that algae possess for more efficient lipid production. Metabolic

engineering through the use of genetic manipulation should be undertaken in order to

optimize the production of algal oils. Large scale culture systems must be designed in

order to allow for the maximum yield of lipids from algal strains. Ways to reduce the

cost and energy consumption associated with the processing of algal biomass must also

be explored. Methods for efficient lipid extraction from algal biomass must be designed

in order to make the process feasible (16).

Current Status

Valcent has developed the vertical algae technology (VAT) technology that mass

produces algae oil that is suitable for refining into biodiesel. Valcent has commissioned a

commercial scale bioreactor pilot project in El Paso, Texas. The VAT bioreactor system

can be deployed on non-arable land. Since VAT is a closed circuit process, much less

water is needed as compared to an open pond system. In addition, the system does not

have large labor costs and does not use fossil fuel burning equipment (21).

Solix Biofuels which was founded in 2006 is in the process of developing reactors

for the growth of microalgae. The technology has the potential to be useful for closed

circle recycling of carbon dioxide emissions from power stations (22).

Another company, PetroSun BioFuels, a subsidiary of the oil company PetroSun,

has launched a program that grows algae in open saltwater ponds that cover 1100 acres in

Rio Hondo, Texas. The aim of the project which was launched in April 2008 is to

produce 4.4 million gallons of algal oil per year (22).

Chevron has agreed to collaborate with the Department of Energy’s National

Renewable Energy Laboratory (NREL) in order to explore the possible production of

biofuels from algae. In addition, Shell has partnered with HR Biopetroleum in an

exploratory project. HR Biopetroleum currently grows algae in ponds on the coast of

Hawaii (22).

A Los Angeles company, OriginOil, is working to develop a large scale reactor to

produce biofuel from algae or to consume carbon dioxide emission. Important issues in

the cultivation of the algae for oil production are a calm fluid setting, an evenly

distributed light source, and an efficient method to breakdown the cell wall of the algae in

order to extract the oil. The company’s Helix BioReactor is a system designed to grow

algae. It has a rotating vertical shaft with low energy light bars set in a spiral pattern to

provide the algae multiple growth layers. Another patented technology that is called

quantum fracturing provides carbon dioxide and other nutrients to the algae through the

bioreactor by micron sized bubbles. The advantage of this system is that it creates a

quick and even distribution while keeping the waters calm. The goal of the company is

to create a modular, scalable reactor system for the production of algae (23).

In the July 20, 2009 issue of Chemical and Engineering News it was reported that

ExxonMobil is investing as much as $600 million to partner with Synthetic Genomics

Inc. (SGI) in the development of algae derived biofuels. J. Craig Venter, a well known

pioneer in genomics research, is cofounder of SGI (26). The money invested by

ExxonMobil will be spent over a period of five to six years on research and development.

It is interesting to note that there are still many issues yet to be decided. The specific

organism that will be used has not yet been decided. In addition, the use of open ponds

or closed bioreactors for the growth of the algae has not been determined (24). Figure 4

shows an artist’s rendition of an algae farm.

Figure 4. Artist’s rendition of algae biofuel farm (27).

Enhancement of Economic Feasibility of Biofuels from Microalgae

As shown in table 4, there are a number of high value bioproducts that can be

extracted from microalgae. One possible method to increase the economical feasibility of

microalgal biofuel production is to coproduce high value products along with the biofuel.

This would conceptually involve sequentially cultivating microalgae in a farming facility

to mitigate CO2 levels, then extracting bioreactive products from the harvested algal

biomass, thermally processing the biomass, extracting high value chemicals, and then

processing the biofuel for different applications. This has the potential to significantly

enhance the cost effectiveness of microalgal biofuel production (25).

Table 4. Some High-Value Bioproducts Extracted from Microalgae

Product group Applications Examples (producer)

Phycobiliproteins carotenoids Polyunsaturated fatty acids (PUFAs) Vitamins

a Heterotrophic growth.

Pigments, cosmetics, pro vitamins, pigmentation

Food additive, nutraceutics

Nutrition

Phycocyanin (Spirulina platensis)

β carotene (Dunaliella salina) astaxanthin and leutin

(Haematococcus pluvialis)

Eicosapentaenoic acid EPA) (Chlorella minutissima)

docosahexaenoic acid (DHA) (Schizochytrium sp.)

Arachidonic acid (AA) (Parietochlorisincise)

Biotin (Euglena gracilis) α-tocopherol (Vitamin E)

(Euglena gracilisa) ascorbic acid (Vitamin C) (Prototheca moriformis, a

Chlorella spp. a)

(25)

Bio-oil and Bio-syngas

When any type of biomass is processed under conditions of high temperature in

the absence of oxygen, the products that are produced are found in three phases – the

vapor phase, the liquid phase, and the solid phase. Bio-oil is a complex mixture of the

liquid phase. It has been found that the overall energy to biomass ratio of a well

controlled pyrolytic process could produce up to 95.5% bio oils and syngas (25). Syngas

is an abbreviation for synthesis gas. This gas is produced form the gasification of a

carbon containing fuel to a gaseous produce that has some heating value (28).

It has been shown that bio-oils are suitable for powering external combustion and

internal combustion engines or for use by co-firing with fossil diesel or natural gas.

Unfortunately, bio-oils have several undesirable features like a high oxygen content, low

heat content, high viscosity at low temperature, and chemical instability. Research to

overcome these obstacles is ongoing. Work by a group in China has shown that

hydrogen can be reliably produced by steam-reforming bio-oil. It has also been

determined that microalgal biomass produces a higher quality bio-oil than biomass from

other sources (25).

Hydrogen from Algae

Hydrogen is another possible fuel source that has been considered as an

alternative to gasoline. The advantage of hydrogen is that water is the only by-product of

the reaction of hydrogen with oxygen, but the difficulty has been finding a viable source

of hydrogen. One possible source is the green alga, Chlamydomonas reinhardtii which is

found around the world as green pond scum. This alga has the potential to produce large

amounts of hydrogen because it can directly split water into hydrogen and oxygen using

the enzyme, hydrogenase. It is thought that this alga evolved to take advantage of very

different environments. In an aerobic environment with plenty of sunlight, the alga

undergoes photosynthesis. However If the alga is forced to live in an anaerobic

environment or is deprived of an essential nutrient like sulfur, it switches to another

mechanism of metabolism and produces hydrogen instead (9, 25)

Scientists have discovered that when the algae are deprived of essential sulfate

salts, they no longer maintain the protein complex necessary for the production of oxygen

photosynthetically and instead switch to the hydrogen-producing metabolic pathway.

However, there are problems that must be resolved before large amounts of hydrogen can

be produced. The alga cannot grow in a sulfur deprived condition for very long before it

needs to revert to the oxygen producing mode. It has been reported that the algae can

grow for 4 days before it needs to return to normal metabolic pathways. During the

period of sulfate deprivation, the algae were found to produce 1.23 x 10-4 moles of

hydrogen for every liter of growing medium at a temperature of 25oC and a pressure of 1

atmosphere (9). The optimal growth temperature for most species of algae is between

20oC to 30oC (29). Much more work is necessary in order to make algae a viable method

for the production of hydrogen.

Conclusion

Microalgal farming has the potential to be combined with flue gas CO2 mitigation

and wastewater treatment. It can also use seawater as a medium when marine microalgal

species are utilized which mitigates the problem of freshwater shortages. In addition,

there is much potential for cost savings when the production of novel products for use in

medicine, food, and cosmetics are coupled with the production of biofuels (25).

Technological developments which include advances in photobioreactor design,

microalgal biomass harvesting, drying and other downstream processing technologies are

important areas that need to be addressed in order to effectively implement the use of

biofuel from algae as a replacement for fossil fuels. In light of the fact that microalgae

are the most efficient primary producers of biomass, it is very likely that they will

eventually become one of the most important alternative energy sources (25).

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